(a) Field of the Invention
The present disclosure relates to a semiconductor device and, in particular, to a method for forming a plug inside a contact hole of the semiconductor device for coupling upper and lower metal wirings, which is capable of reducing interface contact resistance for coupling upper and lower metal wirings.
(b) Description of the Related Art
As the linewidth specified by a design rule decreases, the density and/or integration of devices in a semiconductor device fabrication process increases. In most, if not all, advanced semiconductor devices, a plug structure is utilized. The plug is generally formed by sequentially depositing an interlayer dielectric layer on a lower layer of metal (e.g., copper) wiring, forming a contact hole partly exposing the lower metal wiring, and filling the contact hole with a conductive layer to form the plug, whereby the plug electrically connects the upper and lower metal wirings.
However, when the contact hole for connecting the upper and lower metal wirings is formed, there has been a problem in that polymers containing atoms such as carbon (C) are produced due to the photoresist and/or the etching gas. Also, if the lower metal wiring is exposed through the contact hole, the surface of the exposed metal wiring chemically may react with the oxygen in the atmosphere so as to form a natural oxide layer on the surface of the lower metal wiring. That is, if a lower metal wiring formed out of copper is exposed to the atmosphere, a copper oxide (CuO) is formed on the surface of the exposed copper wiring, which increases the interface contact resistance.
In order to remove the carbon-containing polymer and/or the natural oxide layer formed on the surface of the lower metal wiring, one may perform a baking process in a vacuum state, a sputtering process using argon (Ar) gas, or an annealing process using nitrogen or oxygen. However, sputtering with argon (Ar) gas and annealing with nitrogen and/or oxygen cause problems as following. Firstly, the baking process in vacuum can remove carbon-containing polymers, but not the natural oxide layer formed on the surface of the lower metal wiring which increases the interface resistance. Secondly, sputtering using argon gas can completely remove the natural oxide layer, but it causes leakage current caused by argon ions penetrating the lower metal wiring. Finally, annealing using the nitrogen and oxygen can completely remove the natural oxide layer but not carbon-containing polymers produced while forming the contact hole, resulting in the increase of the device resistance.
U.S. Pat. No. 6,069,073 discloses a method for annealing the surface of refractory metal in a gaseous atmosphere of NH3.